Oxygen-scavenging compositions and articles

ABSTRACT

Oxygen-scavenging compositions comprising an oxidizable metal component, an electrolyte component and a solid, non-electrolytic, acidifying component and, optionally, a water-absorbant binder and/or a polymeric resin exhibit good oxygen-scavenging performance with improved oxidation efficiency relative to compositions containing oxidizable metal component and either an electrolyte or acidifying component but not both. The compositions can be used as an oxygen absorbent in packets or in combination with thermoplastic resins melt-fabricated into a wide variety of oxygen-scavenging packaging articles.

This is a continuation-in-part of application Ser. No. 08/249,758 filedMay 25, 1994 now abandoned, which is a divisional of application Ser.No. 08/092,722 filed Jul. 16, 1993, now abandoned.

FIELD OF THE INVENTION

This invention relates to oxygen-scavenging compositions having utilityin packaging and other applications.

BACKGROUND OF THE INVENTION

Products sensitive to oxygen, particularly foods, beverages andmedicines, deteriorate or spoil in the presence of oxygen. One approachto reducing these difficulties is to package such products withpackaging materials containing at least one layer of a so-called"passive" gas barrier film that can act as a physical barrier totransmission of oxygen but does not react with oxygen. Films obtainedfrom ethylene vinyl alcohol copolymer (EVOH) or polyvinylidenedichloride (PVDC) are commonly used for this purpose due to theirexcellent oxygen barrier properties. By physically blocking transmissionof oxygen, these barrier films can maintain or substantially maintaininitial oxygen levels within a package. Because passive barrier filmscan add cost to a packaging construction and do not reduce levels ofoxygen already present in the packaging construction, however, there isa need for effective, lower cost alternatives and improvements.

An approach to achieving or maintaining a low oxygen environment insidea package is to use a packet containing an oxygen absorbent material.The packet, also sometimes referred to as a pouch or sachet, is placedin the interior of the package along with the product. Sakamoto et al.discloses oxygen absorbent packets in Japan Laid Open Patent ApplicationNo. 121634/81 (1981). A typical ingredient used in the oxygen scavengercarried in the packet is reduced iron powder which can react with oxygento form ferrous oxide or ferric oxide, as disclosed in U.S. Pat. No.4,856,650. Also, it is known to include in the packet, along with iron,a reaction promoter such as sodium chloride, and a water-absorbingagent, such as silica gel, as described in U.S. Pat. No. 4,992,410.Japan Laid Open Patent Application No. 82-24634 (1982) discloses anoxygen absorber composition comprising 100 parts by weight (pbw) ironpowder, 2 to 7 pbw ammonium chloride, 8 to 15 pbw aqueous acid solutionand 20 to 50 pbw of a slightly water soluble filler such as activatedclay. Japan Laid Open Patent Application No. 79-158386 (1979) disclosesan oxygen arresting composition comprising a metal, such as iron, copperor zinc, and optionally, a metal halide such as sodium chloride or zincchloride at a level of 0.001 to 100 pbw to 1 pbw of metal and a fillersuch as clay at a level of 0.01 to 100 pbw to 1 pbw of metal.

Although oxygen absorbent or scavenger materials used in packets canreact chemically with oxygen in the package, also sometimes referred toas "headspace oxygen", they do not prevent external oxygen frompenetrating into the package. Therefore, it is common for packaging inwhich such packets are used to include additional protection such aswrappings of passive barrier films of the type described above. Thisadds to product costs. With many easy-to-prepare foods, anotherdifficulty with oxygen scavenger packets is that consumers maymistakenly open them and consume their contents together with the food.Moreover, the extra manufacturing step of placing a packet into acontainer can add to the cost of the product and slow production.Further, oxygen absorbent packets are not useful with liquid products.

In view of these disadvantages and limitations, it has been proposed toincorporate directly into the walls of a packaging article a so-called"active" oxygen absorber, i.e., one that reacts with oxygen. Becausesuch a packaging article is formulated to include a material that reactswith oxygen permeating its walls, the packaging is said to provide an"active-barrier" as distinguished from passive barrier films which blocktransmission of oxygen but do not react with it. Active-barrierpackaging is an attractive way to protect oxygen-sensitive productsbecause it not only can prevent oxygen from reaching the product fromthe outside but also can absorb oxygen present within a container.

One approach for obtaining active-barrier packaging is to incorporate amixture of an oxidizable metal (e.g., iron) and an electrolyte (e.g.,sodium chloride) into a suitable resin, melt process the result intomonolayer or multilayer sheets or films and form the resulting oxygenscavenger-containing sheets or films into rigid or flexible containersor other packaging articles or components. This type of active-barrieris disclosed in Japan Laid Open Patent Application No. 56-60642 (1981),directed to an oxygen-scavenging sheet composed of a thermoplastic resincontaining iron, zinc or copper and a metal halide. Disclosed resinsinclude polyethylene and polyethylene terephthalate. Sodium chloride isthe preferred metal halide. Component proportions are such that 1 to 500parts metal halide are present per 100 parts resin and 1 to 200 partsmetal halide are present per 100 parts metal. Similarly, U.S. Pat. No.5,153,038 discloses plastic multilayer vessels of various layerstructures formed from a resin composition formed by incorporating anoxygen scavenger, and optionally a water absorbing agent, in a gasbarrier resin. The oxygen scavenger can be a metal powder such as iron,low valence metal oxides or reducing metal compounds. The oxygenscavenger can be used in combination with an assistant compound such asa hydroxide, carbonate, sulfite, thiosulfite, tertiary phosphate,secondary phosphate, organic acid salt or halide of an alkali metal oralkaline earth metal. The water absorbing agent can be an inorganic saltsuch as sodium chloride, calcium chloride, zinc chloride, ammoniumchloride, ammonium sulfate, sodium sulfate, magnesium sulfate, disodiumhydrogenphosphate, sodium dihydrogenphosphate, potassium carbonate orsodium nitrate. The oxygen scavenger can be present at 1 to 1000 weight% based on weight of the barrier resin. The water absorbing agent can bepresent at 1 to 300 weight % based on weight of the barrier resin.

One difficulty with scavenger systems incorporating an oxidizable metal(e.g., iron) and a metal halide (e.g., sodium chloride) into athermoplastic layer is the inefficiency of the oxidation reaction. Toobtain sufficient oxygen absorption in active-barrier packaging, highloadings of scavenger composition are often used. This typicallyrequires that sheets, films and other packaging layer or wall structurescontaining a scavenging composition be relatively thick. This, in turn,contributes to cost of the packaging material and may precludeattainment of thin packaging films having adequate oxygen-scavengingcapabilities.

Another oxygen-scavenging composition, disclosed in U.S. Pat. No.4,104,192, comprises a dithionite and at least one compound having waterof crystallization or water of hydration. Listed among these compoundsare various hydrated sodium salts, including carbonate, sulfate, sulfiteand phosphates; sodium pyrophosphate decahydrate is specificallymentioned. As disclosed in Table 1, Example 1 of the patent, sodiumpyrophosphate decahydrate was the least effective of the compoundstested.

Thus, while a variety of approaches to maintaining or reducing oxygenlevels in packaged items have been advanced, there remains a need forimproved oxygen-scavenging compositions and packaging materialsutilizing the same.

An object of the present invention is to provide improvedoxygen-scavenging compositions and packaging. Another object is toprovide low cost, oxygen-scavenging compositions of improved efficiency.Another object is to provide oxygen-scavenging compositions that can beused effectively, even at relatively low levels, in a wide range ofactive-barrier packaging films and sheets, including laminated andcoextruded multilayer films and sheets. Another object is to provideactive-barrier packaging containers that can increase the shelf-life ofoxygen-sensitive products by slowing the passage of external oxygen intothe container, by absorbing oxygen present inside the container or both.Other objects will be apparent to those skilled in the art.

SUMMARY OF THE INVENTION

These objects can be attained according to the invention by providingoxygen-scavenging compositions comprising at least one oxidizable metalcomponent, at least one electrolyte component and at least one solid,non-electrolytic acidifying component. Optionally, a water-retentivebinder and/or polymeric resin can be included in the composition, ifdesired. For particularly efficient oxygen absorption and cost effectiveformulations, the oxidizable metal component comprises iron, theelectrolyte component comprises sodium chloride and the solid,non-electrolytic, acidifying component comprises sodium acidpyrophosphate. In one embodiment, the invented compositions are providedin the form of a powder or granules for use in packets. In anotherembodiment, the compositions include or are added to a thermoplasticresin and are used in fabrication of articles by melt processingmethods. Concentrates comprising the compositions or their componentsand at least one thermoplastic resin also are provided and offeradvantages in melt processing operations. The invented compositions alsoare provided in the form of packaging structures and components thereof.

As used herein, the term "electrolyte compound" means a compound whichsubstantially dissociates in the presence of water to form positive andnegative ions. By "solid, non-electrolytic acidifying component" or,simply, "acidifying component," is meant a component comprising amaterial which is normally solid and which, in dilute aqueous solution,has a pH less than 7 and disassociates only slightly into positive andnegative ions.

DESCRIPTION OF THE INVENTION

The invented compositions are oxygen-scavenging compositions thatexhibit improved oxygen-absorption efficiency relative to known,oxidizable metal-electrolyte systems, such as iron and sodium chloride,as a result of inclusion in the compositions of a non-electrolytic,acidifying component. In the presence of moisture, the combination ofthe electrolyte and the acidifying components promotes reactivity ofmetal with oxygen to a greater extent than does either alone.Consequently, oxygen absorption efficiency of the invented compositionsis greater than that of known compositions. For a given weight ofoxygen-scavenging composition, the invented compositions provide greaterscavenging capability than conventional materials, other things beingequal. Alternatively, less of the invented composition is needed toprovide a given level of oxygen-scavenging capability than ifconventional materials are used, other things being equal.

Advantageously, when incorporated into thermoplastic resins used formaking of packaging articles and components, the improved efficiency ofthe invented compositions can lead to reductions in not onlyoxygen-scavenger usage but, also, resin usage because the lower loadinglevels permitted by the invented compositions facilitate downgauging tothinner or lighter weight packaging structures.

Another advantage of the invented compositions when used in fabricationof articles by melt processing is that one or more of the components ofthe composition can be provided in the form of a concentrate in athermoplastic resin, thereby facilitating convenient use of thecompositions and tailoring of scavenging compositions to particularproduct requirements.

The oxygen-scavenging composition of the present invention comprises anoxidizable metal component, an electrolyte component, and a solid,non-electrolytic, acidifying component. Optionally, the composition alsocomprises a water-absorbing binder component. The composition can alsocomprise a polymeric resin if desired. The composition can be packagedin an enclosure to form a packet suitable for placement in the interiorof a package. The enclosure can be made from any suitable material thatis permeable to air but not permeable to the components of theoxygen-scavenging composition or the product to be packaged to a degreethat would allow intermingling of the oxygen-scavenging composition withproducts with which it might be packaged. Suitably, the enclosure isconstructed of paper or air-permeable plastic. The composition also canbe incorporated into polymeric resins for use in making fabricatedarticles, for example by melt processing, spraying and coatingtechniques.

Suitable oxidizable metal components comprise at least one metal orcompound thereof capable of being provided in particulate or finelydivided solid form and of reacting with oxygen in the presence of theother components of the composition. For compositions to be used inpackaging applications, the component also should be such that, bothbefore and after reaction with oxygen, it does not adversely affectproducts to be packaged. Examples of oxidizable metals include iron,zinc, copper, aluminum, and tin. Examples of oxidizable metal compoundsinclude ferrous sulfate, cuprous chloride and other iron (II) and copper(I) salts as well as tin (II) salts. Mixtures also are suitable.Oxidizable metal components consisting entirely or mostly of reducediron powder are preferred because they are highly effective in terms ofperformance, cost and ease of use.

The invented compositions also comprise an electrolyte component and asolid, non-electrolytic, acidifying component. These components functionto promote reaction of the oxidizable metal with oxygen. While eithersuch component promotes oxidation in the absence of the other, thecombination is more effective than either alone.

Suitable electrolyte components comprise at least one material thatsubstantially disassociates into positive and negative ions in thepresence of moisture and promotes reactivity of the oxidizable metalcomponent with oxygen. Like the oxidizable metal component, it alsoshould be capable of being provided in granular or powder form and, forcompositions to be used in packaging, of being used without adverselyaffecting products to be packaged. Examples of suitable electrolytecomponents include various electrolytic alkali, alkaline earth andtransition metal halides, sulfates, nitrates, carbonates, sulfites andphosphates, such as sodium chloride, potassium bromide, calciumcarbonate, magnesium sulfate and cupric nitrate. Combinations of suchmaterials also can be used. A particularly preferred electrolytecomponent, both for its cost and performance, is sodium chloride.

The acidifying component comprises a solid, non-electrolytic compoundthat produces an acidic pH, i.e., less than 7, in dilute aqueoussolution. The component disassociates into positive and negative ionsonly slightly in aqueous solution. As with the oxidizable metal andelectrolyte components, for compositions to be used in packagingapplications, the acidifying component should be capable of being usedwithout adversely affecting products to be packaged. For applications inwhich the invented compositions include or are to be used with athermoplastic resin, the acidifying component also should havesufficient thermal stability to withstand melt compounding andprocessing. Suitable materials include various non-electrolytic organicand inorganic acids and their salts. Examples of particular compoundsinclude anhydrous citric acid, citric acid monosodium salt, ammoniumsulfate, magnesium sulfate, disodium dihydrogen pyrophosphate, alsoknown as sodium acid pyrophosphate, sodium metaphosphate, sodiumtrimetaphosphate, sodium hexametaphosphate, citric acid disodium salt,ammonium phosphate, aluminum sulfate, nicotinic acid, aluminum ammoniumsulfate, sodium phosphate monobasic and aluminum potassium sulfate.Combinations of such materials also can be used. Citric acid, sodiumacid pyrophosphate and mixtures thereof work particularly well.

Components of the invented oxygen-scavenging compositions are present inproportions effective to provide oxygen-scavenging effects. Preferably,at least one part by weight electrolyte component plus acidifyingcomponent is present per hundred parts by weight oxidizable metalcomponent, with the weight ratio of electrolyte component to acidifyingcomponent ranging from about 99:1 to about 1:99. More preferably, atleast about 10 parts electrolyte plus acidifying components are presentper 100 parts oxidizable metal component to promote efficient usage ofthe latter for reaction with oxygen. There is no upper limit on theamount of electrolyte plus acidifier relative to metal from thisstandpoint although little or no gain in oxidation efficiency is seenabove about 150 parts per 100 parts metal and economic and processingconsiderations may favor lower levels. In order to achieve anadvantageous combination of oxidation efficiency, low cost and ease ofprocessing and handling, about 30 to about 130 parts electrolyte plusacidifying component per 100 parts metal component are most preferred.

An optional water-absorbing binder can also be included in the inventedcompositions, if desired, to further enhance oxidation efficiency of theoxidizable metal. The binder can serve to provide additional moisturewhich enhances oxidation of the metal in the presence of the promotercompounds. Water-absorbing binders suitable for use generally includematerials that absorb at least about 5 percent of their own weight inwater and are chemically inert. Examples of suitable binders includediatomaceous earth, boehmite, kaolin clay, bentonite clay, acid clay,activated clay, zeolite, molecular sieves, talc, calcined vermiculite,activated carbon, graphite, carbon black, and the like. It is alsocontemplated to utilize organic binders, examples including variouswater absorbent polymers as disclosed in Koyama et al., European PatentApplication No. 428,736. Mixtures of such binders also can be employed.Preferred binders are bentonite clay, kaolin clay, and silica gel. Whenused, the water-absorbent binder preferably is used in an amount of atleast about five parts by weight per hundred parts by weight of theoxidizable metal, electrolyte and acidifying components. Morepreferably, about 15 to about 100 parts per hundred parts metal arepresent as lesser amounts may have little beneficial effect whilegreater amounts may hinder processing and handling of the overallcompositions without offsetting gain in oxygen-scavenging performance.When a binder component is used in compositions compounded intoplastics, the binder most preferably is present in an amount rangingfrom about 10 to about 50 parts per hundred parts metal to enhanceoxidation efficiency at loading levels low enough to ensure ease ofprocessing.

A particularly preferred oxygen-scavenging composition according to theinvention comprises iron powder, sodium chloride and sodium acidpyrophosphate, with about 10 to about 150 parts by weight sodiumchloride plus sodium acid pyrophosphate being present per hundred partsby weight iron and the weight ratio of sodium chloride to sodium acidpyrophosphate being about 10:90 to about 90:10. Optionally, up to about100 parts by weight water absorbing binder per hundred parts by weightof the other components also are present. Most preferably, thecomposition comprises iron powder, about 5 to about 100 parts sodiumchloride and about 5 to about 70 parts sodium acid pyrophosphate perhundred parts iron and up to about 50 parts binder per hundred parts ofthe other components.

According to another aspect of this invention, there is provided anoxygen-scavenger resin composition comprising at least one plastic resinand the above-described oxygen-scavenging composition, with or withoutthe water-absorbent binder component.

Any suitable polymeric resin into which an effective amount of theoxygen-scavenging composition of this invention can be incorporated andthat can be formed into a laminar configuration, such as film, sheet ora wall structure, can be used as the plastic resin in the compositionsaccording to this aspect of the invention. Thermoplastic and thermosetresins can be used. Examples of thermoplastic polymers includepolyamides, such as nylon 6, nylon 66 and nylon 612, linear polyesters,such as polyethylene terephthalate, polybutylene terephthalate andpolyethylene naphthalate, branched polyesters, polystyrenes,polycarbonate, polymers of unsubstituted, substituted or functionalizedolefins such as polyvinyl chloride, polyvinylidene dichloride,polyacrylamide, polyacrylonitrile, polyvinyl acetate, polyacrylic acid,polyvinyl methyl ether, ethylene vinyl acetate copolymer, ethylenemethyl acrylate copolymer, polyethylene, polypropylene,ethylene-propylene copolymers, poly(1 -hexene), poly(4-methyl-1-pentene), poly(1 -butene), poly(3-methyl-1-butene),poly(3-phenyl-1-propene) and poly(vinylcyclohexane). Homopolymers andcopolymers are suitable as are polymer blends containing one or more ofsuch materials. Thermosetting resins, such as epoxies, oleoresins,unsaturated polyester resins and phenolics also are suitable.

Preferred polymers are thermoplastic resins having oxygen permeationcoefficients greater than about 2×10⁻¹² cc-cm/cm² -sec-cm Hg as measuredat a temperature of 20° C. and a relative humidity of 0% because suchresins are relatively inexpensive, easily formed into packagingstructures and, when used with the invented oxygen-scavengingcompositions, can provide a high degree of active barrier protection tooxygen-sensitive products. Examples of these include polyethyleneterephthalate and polyalpha-olefin resins such as high, low and linearlow density polyethylene and polypropylene. Even relatively low levelsof oxygen-scavenging composition, e.g., about 5 to about 15 parts perhundred parts resin, can provide a high degree of oxygen barrierprotection to such resins. Among these preferred resins, permeability tooxygen increases in the order polyethylene terephthalate, polypropylene,high density polyethylene, linear low density polyethylene and lowdensity polyethylene, other things being equal. Accordingly, for suchpolymeric resins, oxygen scavenger loadings for achieving a given levelof oxygen barrier effectiveness increase in like order, other thingsbeing equal.

In selecting a thermoplastic resin for use or compounding with theoxygen-scavenging composition of the invention, the presence of residualantioxidant compounds in the resin can be detrimental to oxygenabsorption effectiveness. Phenol-type antioxidants and phosphite-typeantioxidants are commonly used by polymer manufacturers for the purposeof enhancing thermal stability of resins and fabricated productsobtained therefrom. Specific examples of these residual antioxidantcompounds include materials such as butylated hydroxytoluene,tetrakis(methylene(3,5-di-t-butyl-4-hydroxyhydro-cinnamate)methane andtriisooctyl phosphite. Such antioxidants are not to be confused with theoxygen scavenger components utilized in the present invention.Generally, oxygen absorption of the scavenger compositions of thepresent invention is improved as the level of residual antioxidantcompounds is reduced. Thus, commercially available resins containing lowlevels of phenol-type or phosphite-type antioxidants, preferably lessthan about 1600 ppm, and most preferably less than about 800 ppm, byweight of the resin, are preferred (although not required) for use inthe present invention. Examples are Dow Chemical Dowlex 2032 linear lowdensity polyethylene (LLDPE); Union Carbide GRSN 7047 LLDPE; GoodyearPET "Traytuf" 9506; and Eastman PETG 6763. Measurement of the amount ofresidual antioxidant can be performed using high pressure liquidchromatography.

When used in combination with resins, the oxidizable metal, electrolyteand acidifying components of the invented oxygen-scavengingcompositions, and any optional water-absorbent binder that may be used,are used in particulate or powder form. Particle sizes of at least 50mesh or smaller are preferred to facilitate melt-processing of oxygenscavenger thermoplastic resin formulations. For use with thermosetresins for formation of coatings, particle sizes smaller than thethickness of the final coating are employed. The oxygen scavenger can beused directly in powder or particulate form, or it can be processed, forexample by melt compounding or compaction-sintering, into pellets tofacilitate further handling and use. The mixture of oxidizable metalcomponent, electrolyte component, acidifying component and optionalwater-absorbent binder can be added directly to a thermoplastic polymercompounding or melt-fabrication operation, such as in the extrusionsection thereof, after which the molten mixture can be advanced directlyto a film or sheet extrusion or coextrusion line to obtain monolayer ormultilayer film or sheet in which the amount of oxygen-scavengingcomposition is determined by the proportions in which the compositionand resin are combined in the resin feed section of theextrusion-fabrication line. Alternatively, the mixture of oxidizablemetal component, electrolyte component, acidifying component andoptional binder can be compounded into masterbatch concentrate pellets,which can be further let down into packaging resins for furtherprocessing into extruded film or sheet, or injection molded articlessuch as tubs, bottles, cups, trays and the like.

The degree of mixing of oxidizable metal, electrolyte and acidifyingcomponents and, if used, optional binder component has been found toaffect oxygen absorption performance of the oxygen-scavengingcompositions, with better mixing leading to better performance. Mixingeffects are most noticeable at low electrolyte plus acidifyingcomponents to oxidizable metal component ratios and at very low and veryhigh acidifying component to electrolyte component ratios. Below about10 parts by weight electrolyte plus acidifying components per hundredparts by weight metal component, or when the weight ratio of either theelectrolyte or acidifying component to the other is less than about10:90, the oxygen scavenger components are preferably mixed by aqueousslurry mixing followed by oven drying and grinding into fine particles.Below these ratios, mixing by techniques suitable at higher ratios, suchas by high-intensity powder mixing, as in a Henschel mixer or a Waringpowder blender, or by lower intensity mixing techniques, as in acontainer on a roller or tumbler, may lead to variability in oxygenuptake, particularly when the compositions are incorporated intothermoplastic resins and used in melt processing operations. Otherthings being equal, it has been found that oxygen-scavengingcompositions prepared by slurry mixing have the highest oxygenabsorption efficiency or performance, followed in order by compositionsprepared using high intensity solids mixers and roller/tumbler mixingtechniques.

Other factors that may affect oxygen absorption performance of theinvented oxygen-scavenging compositions include surface area of articlesincorporating the compositions, with greater surface area normallyproviding better oxygen absorption performance. The amount of residualmoisture in the water-absorbant binder, if used, also can affectperformance with more moisture in the binder leading to better oxygenabsorption performance. However, there are practical limits on theamount of moisture that should be present in the binder because too muchcan cause premature activation of the oxygen-scavenger composition aswell as processing difficulties and poor aesthetics in fabricatedproducts. When incorporated into thermoplastic resins and used forfabrication of articles by melt processing techniques, the nature of theresin also can have a significant effect. Thus when the inventedoxygen-scavenging compositions are used with amorphous and/or oxygenpermeable polymers such as polyolefins or amorphous polyethyleneterephthalate, higher oxygen absorption is seen than when thecompositions are used with crystalline and/or oxygen barrier polymerssuch as crystalline polyethylene terephthalate and EVOH.

When used with thermoplastic resins, the oxygen-scavenging compositionscan be incorporated directly into the resin in amounts effective toprovide the desired level of oxygen-scavenging ability. When so-used,preferred oxygen scavenger levels will vary depending on the choice ofresin, configuration of the article to be fabricated from the resin andoxygen-scavenging capability needed in the article. Use of resins withlow inherent viscosity, e.g., low molecular weight resins, normallypermits higher loadings of scavenger composition without loss ofprocessability. Conversely, lesser amounts of oxygen scavenger mayfacilitate use of polymeric materials having higher viscosities.Preferably, at least about 2 parts by weight oxygen-scavengingcomposition are used per 100 parts by weight resin. Loading levels aboveabout 200 parts per hundred parts resin generally do not lead to gainsin oxygen absorption and may interfere with processing and adverselyaffect other product properties. More preferably, loading levels ofabout 5 to about 150 parts per hundred are used to obtain goodscavenging performance while maintaining processibility. Loading levelsof about 5 to about 15 parts per hundred are particularly preferred forfabrication of thin films and sheets.

Preferred oxygen-scavenger resin compositions for fabrication ofpackaging articles comprise at least one thermoplastic resin and about 5to about 50 parts by weight oxygen-scavenging composition per hundredparts by weight resin, with the oxygen-scavenging composition comprisingiron powder, sodium chloride and sodium acid pyrophosphate. Morepreferably, about 30 to about 130 parts by weight sodium chloride andsodium acid pyrophosphate per hundred parts by weight iron are presentin the scavenging composition and the weight ratio of sodium chloride tosodium acid pyrophosphate is about 10:90 to about 90:10. Up to about 50parts by weight water-absorbant binder per hundred parts by weight ofresin and oxygen scavenger also can be included. Especially preferredcompositions of this type comprise polypropylene, high, low or linearlow density polyethylene or polyethylene terephthalate as the resin,about 5 to about 30 parts by weight oxygen scavenger per hundred partsby weight resin, about 5 to about 100 parts by weight sodium chlorideand about 5 to about 70 parts by weight sodium acid pyrophosphate perhundred parts by weight iron and up to about 50 parts, by weight binderper hundred parts by weight iron plus sodium chloride plus sodium acidpyrophosphate.

While the oxygen-scavenging composition and resin can be used in anon-concentrated form for direct fabrication of scavenging sheets orfilms (i.e., without further resin dilution), it also is beneficial touse the oxygen-scavenging composition and resin in the form of aconcentrate. When so-used, the ability to produce a concentrate with lowmaterials cost weighs in favor of relatively high loadings of scavengerthat will still permit successful melt compounding, such as by extrusionpelletization. Thus concentrate compositions according to the inventionpreferably contain at least about 10 parts by weight oxygen-scavengingcomposition per hundred parts by weight resin and more preferably about30 to about 150 parts per hundred. Suitable resins for suchoxygen-scavenging concentrate compositions include any of thethermoplastic polymer resins described herein. Low melt viscosity resinsfacilitate use of high scavenger loadings and typically are used insmall enough amounts in melt fabrication of finished articles that thetypically lower molecular weight of the concentrate resin does notadversely affect final product properties. Preferred carrier resins arepolypropylene, high density, low density and linear low densitypolyethylenes and polyethylene terephthalate. Preferred among those arepolypropylenes having melt flow rates of about 1 to about 40 g/10 min,polyethylenes having melt indices of about 1 to about 20 g/10 min andpolyethylene terephthalates having inherent viscosities of about 0.6 toabout 1 in phenol/trichloroethane.

It also is contemplated to utilize various components of theoxygen-scavenging composition or combinations of such components to formtwo or more concentrates that can be combined with a thermoplastic resinand fabricated into an oxygen-scavenging product. An advantage of usingtwo or more concentrates is that the electrolyte and acidifyingcomponents can be isolated from the oxidizable metal until preparationof finished articles, thereby preserving full or essentially full oxygenscavenging capability until actual use and permitting lower scavengerloadings than would otherwise be required. In addition, separateconcentrates permit more facile preparation of differing concentrationsof the electrolyte and acidifying components and/or water absorbantbinder with the oxidizable metal and also enable fabricators toconveniently formulate a wide range of melt-processible resincompositions in which oxygen scavenging ability can be tailored tospecific end use requirements. Preferred components or combinations ofcomponents for use in separate concentrates are (1) acidifyingcomponent; (2) combinations of oxidizable metal component with waterabsorbing binder component; and (3) combinations of electrolyte andacidifying components.

A particularly preferred component concentrate is a compositioncomprising sodium acid pyrophosphate and a thermoplastic resin. Such aconcentrate can be added in desired amounts in melt fabricationoperations utilizing thermoplastic resin that already contains, or towhich will be added, other scavenging components, such as an oxidizablemetal or combination thereof with an electrolyte, to provide enhancedoxygen scavenging capability. Especially preferred are concentratescontaining about 10 to about 150 parts by weight sodium acidpyrophosphate per hundred parts by weight resin, with polypropylene,polyethylenes and polyethylene terephthalate being most preferredresins.

Polymeric resins that can be used for incorporating theoxygen-scavenging compositions into internal coatings of cans via spraycoating and the like are typically thermoset resins such as epoxy,oleoresin, unsaturated polyester resins or phenolic based materials.

This invention also provides articles of manufacture comprising at leastone melt-fabricated layer incorporating the oxygen-scavengingcompositions as described above. Because of the improved oxidationefficiency afforded by the invented oxygen-scavenging compositions, thescavenger-containing layer can contain relatively low levels of thescavenger. The articles of the present invention are well suited for usein flexible or rigid packaging structures. In the case of rigid sheetpackaging according to the invention, the thickness of theoxygen-scavenging layer is preferably not greater than about 100 mils,and is most preferably in the range of about 10 to about 50 mils. In thecase of flexible film packaging according to the invention, thethickness of the oxygen scavenger layer is preferably not greater thanabout 10 mils and, most preferably, about 0.5 to about 8 mils. As usedherein, the term "mils" is used for its common meaning, i.e.,one-thousandth of an inch. Packaging structures according to theinvention can be in the form of films or sheets, both rigid andflexible, as well as container or vessel walls and liners as in trays,cups, bowls, bottles, bags, pouches, boxes, films, cap liners, cancoatings and other packaging constructions. Both monolayer andmultilayer structures are contemplated.

The oxygen-scavenging composition and resin of the present inventionafford active-barrier properties in articles fabricated therefrom andcan be melt processed by any suitable fabrication technique intopackaging walls and articles having excellent oxygen barrier propertieswithout the need to include layers of costly gas barrier films such asthose based on EVOH, PVDC, metallized polyolefin or polyester, aluminumfoil, silica coated polyolefin and polyester, etc. The oxygen-scavengerarticles of the present invention also provide the additional benefit ofimproved recyclability. Scrap or reclaim from the oxygen-scavengingresin can be easily recycled back into plastic products without adverseeffects. In contrast, recycle of EVOH or PVDC gas barrier films maycause deterioration in product quality due to polymer phase separationand gelation occurring between the gas barrier resin and other resinsmaking up the product. Nevertheless, it also is contemplated to providearticles, particularly for packaging applications, with both active andpassive oxygen barrier properties through use of one or more passive gasbarrier layers in articles containing one or more active barrier layersaccording to the invention. Thus, for some applications, such aspackaging for food for institutional use and others calling for longshelf-life, an oxygen-scavenging layer according to the presentinvention can be used in conjunction with a passive gas barrier layer orfilm such as those based on EVOH, PVDC, metallized polyolefins oraluminum foil.

The present invention is also directed to a packaging wall containing atleast one layer comprising the oxygen-scavenging composition and resindescribed above. It should be understood that any packaging article orstructure intended to completely enclose a product will be deemed tohave a "packaging wall," as that term is used herein, if the packagingarticle comprises a wall, or portion thereof, that is, or is intended tobe, interposed between a packaged product and the atmosphere outside ofthe package and such wall or portion thereof comprises at least onelayer incorporating the oxygen-scavenging composition of the presentinvention. Thus, bowls, bags, liners, trays, cups, cartons, pouches,boxes, bottles and other vessels or containers which are intended to besealed after being filled with a given product are covered by the term"packaging wall" if the oxygen-scavenging composition of the inventionis present in any wall of such vessel (or portion of such wall) which isinterposed between the packaged product and the outside environment whenthe vessel is closed or sealed. One example is where theoxygen-scavenging composition of the invention is fabricated into, orbetween, one or more continuous thermoplastic layers enclosing orsubstantially enclosing a product. Another example of a packaging wallaccording to the invention is a monolayer or multilayer film containingthe present oxygen-scavenging composition used as a cap liner in abeverage bottle (i.e., for beer, wine, fruit juices, etc.) or as awrapping material.

An attractive active-barrier layer is generally understood as one inwhich the kinetics of the oxidation reaction are fast enough, and thelayer is thick enough, that most of the oxygen permeating into the layerreacts without allowing a substantial amount of the oxygen to transmitthrough the layer. Moreover, it is important that this "steady state"condition exist for a period of time appropriate to end use requirementsbefore the scavenger layer is spent. The present invention affords thissteady state, plus excellent scavenger longevity, in economicallyattractive layer thicknesses, for example, less than about 100 mils inthe case of sheets for rigid packaging, and less than about 10 mils inthe case of flexible films. For rigid sheet packaging according to thepresent invention, an attractive scavenger layer can be provided in therange of about 10 to about 30 mils, while for flexible film packaging,layer thicknesses of about 0.5 to about 8 mils are attractive. Suchlayers can function efficiently with as little as about 2 to about 10weight % oxygen scavenger composition based on weight of the scavengerlayer.

In fabrication of packaging structures according to the invention, it isimportant to note that the oxygen-scavenging resin composition of theinvention is substantially inactive with respect to chemical reactionwith oxygen so long as the water activity of the composition is lessthan about 0.2-0.3. In contrast, the composition becomes active forscavenging oxygen when the water activity is at or above about 0.2-0.3.Water activity is such that, prior to use, the invented packagingarticles can remain substantially inactive in relatively dryenvironments without special steps to maintain low moisture levels.However, once the packaging is placed into use, most products will havesufficient moisture to activate the scavenger composition incorporatedin the walls of the packaging article. In the case of a hypotheticalpackaging article according to the invention having an intermediateoxygen-scavenging layer sandwiched between inner and outer layers, thescavenging layer of the structure, in which the oxygen-scavengingcomposition of the present invention is contained, will be active forchemical reaction with oxygen permeating into the scavenging layer ifthe following equation is satisfied: ##EQU1## where: d_(i) is thethickness in mils of the inner layer;

d_(o) is the thickness in mils of the outer layer;

a_(o) is the water activity of the environment outside the packagingarticle (i.e., adjacent the outer layer);

a_(i) is the water activity of the environment inside the packagingarticle (i.e., adjacent the inner layer);

a is the water activity of the scavenging layer;

(WVTR)_(o) is the water vapor transmission rate of the outer layer ofthe packaging wall in gm.mil/100 in. sq. day at 100° F. and 90% RHaccording to ASTM E96; and

(WVTR)_(i) is the water vapor transmission rate of the inner layer ofthe packaging wall in gm.mil/100 in. sq. day at 100°F. and 90% RHaccording to ASTM E96.

For monolayer packaging constructions in which a layer of orincorporating the oxygen-scavenging composition is the only layer of thepackaging wall, the package will be active for oxygen absorptionprovided a_(o) or a_(i) is greater than or equal to about 0.2-0.3.

To prepare a packaging wall according to the invention, anoxygen-scavenging resin formulation is used or the oxygen-scavengingcomposition, or its components or concentrates thereof, is compoundedinto or otherwise combined with a suitable packaging resin whereupon theresulting resin formulation is fabricated into sheets, films or othershaped structures. Extrusion, coextrusion, blow molding, injectionmolding and any other sheet, film or general polymeric melt-fabricationtechnique can be used. Sheets and films obtained from the oxygenscavenger composition can be further processed, e.g. by coating orlamination, to form multilayered sheets or films, and then shaped, suchas by thermoforming or other forming operations, into desired packagingwalls in which at least one layer contains the oxygen scavenger. Suchpackaging walls can be subjected to further processing or shaping, ifdesired or necessary, to obtain a variety of active-barrier end-usepackaging articles. The present invention reduces the cost of suchbarrier articles in comparison to conventional articles which affordbarrier properties using passive barrier films.

As a preferred article of manufacture, the invention provides apackaging article comprising a wall, or combination of interconnectedwalls, in which the wall or combination of walls defines an enclosableproduct-receiving space, and wherein the wall or combination of wallscomprises at least one wall section comprising an oxygen-scavenginglayer comprising (i) a polymeric resin, preferably a thermoplastic resinor a thermoset resin and most preferably a thermoplastic resin selectedfrom the group consisting of polyolefins, polystyrenes and polyesters;(ii) an oxidizable metal preferably comprising at least one memberselected from the group consisting of iron, copper, aluminum, tin andzinc, and most preferably about 1 to about 100 parts iron per hundredparts by weight of the resin; (iii) an electrolyte component and asolid, non-electrolytic, acidifying component which in the presence ofwater has a pH of less than 7, with about 5 to about 150 parts by weightof such components per hundred parts by weight of iron preferably beingpresent and the weight ratio of the acidifying component to electrolytecomponent preferably being about 5/95 to about 95/5; and, optionally, awater-absorbent binder. In such articles, sodium chloride is the mostpreferred electrolyte component and sodium acid pyrophosphate is mostpreferred as the acidifying component, with the weight ratio of sodiumacid pyrophosphate to sodium chloride most preferably ranging from about10/90 to about 90/10.

A particularly attractive packaging construction according to theinvention is a packaging wall comprising a plurality of thermoplasticlayers adhered to one another in bonded laminar contact wherein at leastone oxygen-scavenging layer is adhered to one or more other layers whichmay or may not include an oxygen-scavenging composition. It isparticularly preferred, although not required, that the thermoplasticresin constituting the major component of each of the layers of thepackaging wall be the same, so as to achieve a "pseudo-monolayer". Sucha construction is easily recyclable.

An example of a packaging article using the packaging wall describedabove is a two-layer or three-layer dual ovenable tray made ofcrystalline polyethylene terephthalate ("C-PET") suitable for packagingpre-cooked single-serving meals. In a three-layer construction, anoxygen-scavenging layer of about 10 to 20 mils thickness is sandwichedbetween two non-scavenging C-PET layers of 3 to 10 mils thickness. Theresulting tray is considered a "pseudo-monolayer" because, for practicalpurposes of recycling, the tray contains a single thermoplastic resin,i.e., C-PET. Scrap from this pseudo-monolayer tray can be easilyrecycled because the scavenger in the center layer does not detract fromrecyclability. In the C-PET tray, the outer, non-scavenging layerprovides additional protection against oxygen transmission by slowingdown the oxygen so that it reaches the center layer at a sufficientlyslow rate that most of the ingressing oxygen can be absorbed by thecenter layer without permeating through it. The optional innernon-scavenging layer acts as an additional barrier to oxygen, but at thesame time is permeable enough that oxygen inside the tray may pass intothe central scavenging layer. It is not necessary to use a three layerconstruction. For example, in the above construction, the inner C-PETlayer can be eliminated. A tray formed from a single oxygen scavenginglayer is also an attractive construction.

The pseudo-monolayer concept can be used with a wide range of polymericpackaging materials to achieve the same recycling benefit observed inthe case of the pseudo-monolayer C-PET tray. For example, a packagefabricated from polypropylene or polyethylene can be prepared from amultilayer packaging wall (e.g., film) containing the oxygen-scavengingcomposition of the present invention. In a two-layer construction thescavenger layer can be an interior layer with a non-scavenging layer ofpolymer on the outside to provide additional barrier properties. Asandwich construction is also possible in which a layer ofscavenger-containing resin, such as polyethylene, is sandwiched betweentwo layers of non-scavenging polyethylene. Alternatively, polypropylene,polystyrene or another suitable resin can be used for all of the layers.

Various modes of recycle may be used in the fabrication of packagingsheets and films according to the invention. For example, in the case ofmanufacturing a multilayer sheet or film having a scavenging andnon-scavenging layer, reclaim scrap from the entire multilayer sheet canbe recycled back into the oxygen scavenging layer of the sheet or film.It is also possible to recycle the multilayer sheet back into all of thelayers of the sheet.

Packaging walls and packaging articles according to the presentinvention may contain one or more layers which are foamed. Any suitablepolymeric foaming technique, such as bead foaming or extrusion foaming,can be utilized. For example, a packaging article can be obtained inwhich a foamed resinous layer comprising, for example, foamedpolystyrene, foamed polyester, foamed polypropylene, foamed polyethyleneor mixtures thereof, can be adhered to a solid resinous layer containingthe oxygen-scavenging composition of the present invention.Alternatively, the foamed layer may contain the oxygen-scavengingcomposition, or both the foamed and the non-foamed layer can contain thescavenging composition. Thicknesses of such foamed layers normally aredictated more by mechanical property requirements, e.g. rigidity andimpact strenth, of the foam layer than by oxygen-scavengingrequirements.

Packaging constructions such as those described above can benefit fromthe ability to eliminate costly passive barrier films. Nevertheless, ifextremely long shelf life or added oxygen protection is required ordesired, a packaging wall according to the invention can be fabricatedto include one or more layers of EVOH, nylon or PVDC, or even ofmetallized polyolefin, metallized polyester, or aluminum foil. Anothertype of passive layer which may be enhanced by an oxygen-scavengingresin layer according to the present invention is silica-coatedpolyester or silica-coated polyolefin. In cases where a multilayerpackaging wall according to the invention contains layers of differentpolymeric compositions, it may be preferable to use adhesive layers suchas those based on ethylene-vinyl acetate copolymer or maleatedpolyethylene or polypropylene, and if desired, the oxygen scavenger ofthe present invention can be incorporated in such adhesive layers. It isalso possible to prepare the oxygen-scavenging composition of thepresent invention using a gas barrier resin such as EVOH, nylon or PVDCpolymer in order to obtain a film having both active and passive barrierproperties.

While the focus of one embodiment of the invention is upon theincorporation of the oxygen-scavenging composition directly into thewall of a container, the oxygen-scavenging compositions also can be usedin packets, as a separate inclusion within a packaging article where theintent is only to absorb headspace oxygen.

A primary application for the oxygen-scavenging resin, packaging walls,and packaging articles of the invention is in the packaging ofperishable foods. For example, packaging articles utilizing theinvention can be used to package milk, yogurt, ice cream, cheeses; stewsand soups; meat products such as hot dogs, cold cuts, chicken, beefjerky; single-serving pre-cooked meals and side dishes; homemade pastaand spaghetti sauce; condiments such as barbecue sauce, ketchup,mustard, and mayonnaise; beverages such as fruit juice, wine, and beer;dried fruits and vegetables; breakfast cereals; baked goods such asbread, crackers, pastries, cookies, and muffins; snack foods such ascandy, potato chips, cheese-filled snacks; peanut butter or peanutbutter and jelly combinations, jams, and jellies; dried or freshseasonings; and pet and animal foods; etc. The foregoing is not intendedto be limiting with respect to the possible applications of theinvention. Generally speaking, the invention can be used to enhance thebarrier properties in packaging materials intended for any type ofproduct which may degrade in the presence of oxygen.

Still other applications for the oxygen-scavenging compositions of thisinvention include the internal coating of metal cans, especially foroxygen-sensitive food items such as tomato-based materials, baby foodand the like. Typically the oxygen-scavenging composition can becombined with polymeric resins such as thermosets of epoxy, oleoresin,unsaturated polyester resins or phenolic based materials and thematerial applied to the metal can by methods such as roller coating orspray coating.

The examples provided below are for purposes of illustration and are notintended to limit the scope of invention.

For purposes of the following examples, oxygen scavenging performancewas measured according to an Oxygen Absorption Test performed in a 500ml glass container containing the oxygen-scavenging composition in theform of powder, concentrate pellet or film. Distilled water or anaqueous salt solution in an open vial was placed inside the glasscontainer next to the samples to be tested in order to control therelative humidity in the container. The container was then sealed andstored at the test temperature. The residual oxygen concentration in theheadspace of the container was measured initially and then periodicallyusing a Servomex Series 1400 Oxygen Analyzer. The amount of oxygenabsorbed by the test sample was determined from the change in the oxygenconcentration in the headspace of the glass container. The testcontainer had a headspace volume of about 500 ml and containedatmospheric air so that about 100 ml of oxygen were available forreaction with the iron. Test samples having an iron content of about 0.5gm Fe were tested. For the test system, iron oxidized from metal to FeOhas a theoretical oxygen absorption level of 200 cc O₂ /gm Fe and ironoxidized from metal to Fe₂ O₃ has a theoretical oxygen absorption levelof 300 cc O2/gm Fe. In all of the examples, oxygen scavenger componentpercentages are in weight percents based on total weight of thecompositions, whether film, powder or pellet, tested for oxygenabsorbtion.

EXAMPLE 1

Various powder mixtures of iron powder (SCM Iron Powder A-131); sodiumchloride (Morton pulverized salt, Extra Fine 200); bentonite clay(Whittaker, Clarke & Davis, WCD-670); anhydrous sodium acid pyrophospate("SAP"), Na₂ H₂ P₂ O₇ (Sigma #7758-16-9); sodium pyrophosphatedecahydrate ("SPH"), Na₄ P₂ O₇.10H₂ O (Aldrich 22,136-8) and anhydroussodium pyrophosphate ("SPA"), Na₄ P₂ O₇ (Aldrich 32,246-6) were preparedas described below. Upon water absorption, SAP has a pH of 4 and SPH andSPA each has a pH of 10. The bentonite clay had been dried separatelyovernight at 250° C. in a vacuum oven. The desired weights ofingredients were dry blended in a Waring blender and the blendedingredients were stored under a nitrogen atmosphere. Samples 1-1 and 1-2and comparative samples Comp 1-A through Comp 1-I were tested for oxygenabsorption at test conditions of 168 hr, a relative humidity of 100% anda temperature of 22° C. Results are tabulated below. This Exampledemonstrates that the oxygen-scavenging compositions of this inventionemploying iron, sodium chloride and SAP provide equivalent or betteroxygen absorbing efficiency than compositions of iron and sodiumchloride with or without clay. Comparative compositions with iron,sodium chloride and SPH or SPA exhibit considerably lower oxygenabsorption values. Also, comparative compositions with iron and clay,SAP, SPH or SPA all exhibited very low values of oxygen absorption withno electrolyte compound, sodium chloride, present.

    ______________________________________                                        Powder         Na Cl,         Additive,                                                                            Clay,                                                                              cc O.sub.2                          No.     Fe, %  %       Additive                                                                             %      %    gm Fe                               ______________________________________                                        1-1     50     37.5    SAP    12.5   0    204                                 1-2     44.4   33.3    SAP    11.1   11.1 169                                 Comp 1-A                                                                              100    0       --     0      0     5                                  Comp 1-B                                                                              57.1   42.9    --     0      0    202                                 Comp 1-C                                                                              50     37.5    --     0      12.5 204                                 Comp 1-D                                                                              50     37.5    SPH    12.5   0    74                                  Comp 1-E                                                                              50     37.5    SPA    12.5   0    44                                  Comp 1-F                                                                              80     0       --     0      20   39                                  Comp 1-G                                                                              80     0       SAP    20     0    17                                  Comp 1-H                                                                              80     0       SPH    20     0     2                                  Comp 1-I                                                                              80     0       SPA    20     0     2                                  ______________________________________                                    

EXAMPLE 2

A dry-mix preparation of oxygen scavenger ingredients was carried out inthe following manner: Iron powder (SCM Iron Powder A-131); sodiumchloride (Morton pulverized salt, Extra Fine 200); bentonite clay(Whittaker, Clarke & Davis, WCD-670) and anhydrous sodium acidpyrophosphate (SAP), Na₂ H₂ P₂ O₇ (Sigma #7758-16-9) were dry blended ina Waring blender at a weight ratio of Fe:NaCl:bentonite clay:Na₂ H₂ P₂O₇ of 4:3:1:2. The bentonite clay had been dried separately overnight at250° C. in a vacuum oven. The blended oxygen scavenger ingredients werestored under nitrogen. A concentrate of oxygen-scavenger and polymerresin was prepared from a 50/50 weight ratio of linear low densitypolyethylene granules (GRSN 7047, Union Carbide) and the oxygenscavenger composition by tumble mixing in a bucket/bottle roller for tenminutes to obtain a homogeneous mixture. The resultant powder blend wasfed directly to the hopper of a 19 mm conical corotating twin-screwextruder equipped with a strand die. The zone temperatures of theextruder barrel were set as follows: zone 1--215° C., zone 2--230° C.,zone 3--230° C., and strand die--230° C. The extrudate was cooled withroom-temperature water in a water bath and chopped into pellets with apelletizer. The pellets were dried overnight at 100° C. in a vacuum ovenand stored under nitrogen.

EXAMPLE 3

Low density polyethylene oxygen-scavenging films were prepared byextruding a mixture containing 80 parts by weight (pbw) low densitypolyethylene pellets (DOW 526 I, Dow Chemical) having a nominal oxygenpermeation coefficient (OPC) of 1.5--2.1×10⁻¹⁰ cc-cm/cm² -sec-cm Hg, asmeasured at a temperature of 20° C. and a relative humidity of 0%, and20 pbw of an oxygen-scavenging composition in the form of a concentrateprepared according to the procedure described in Example 2. Theconcentrates contained various amounts of iron, sodium chloride,bentonite clay and SAP as tabulated below with the weight ratio ofsodium chloride to iron maintained at about 0.75:1. Films were preparedusing a Haake Rheomex 245 single screw extruder (screw diameter--19 mm;UD ratio--25:1). The zone temperatures of the extruder barrel were setas follows: zone 1--245° C., zone 2--250° C., zone 3--250° C. anddie--230° C. Nominal thicknesses of the extruded films were 5 mils.Tabulated below is the amount of oxygen absorbed by each of the filmsamples as measured by the Oxygen Absorption Test described above attest conditions of 168 hr, a relative humidity of 100% and a temperatureof 22° C. This example demonstrates that at a given weight ratio ofsodium chloride to iron, addition of SAP significantly increases theoxygen absorption of the low density polyethylene oxygen scavengingfilm.

    ______________________________________                                                                                cc O.sub.2                            Film No.                                                                             Iron, %   NaCl, %  SAP, %  Clay, %                                                                             gm Fe                                 ______________________________________                                        3-1    4.00      3.00     2.00    1.00  92                                    3-2    4.44      3.33     1.11    1.11  50                                    3-3    4.71      3.53     0.59    1.18  51                                    ______________________________________                                    

EXAMPLE 4

Low density polyethylene oxygen scavenging films were prepared by thesame procedure as described in Example 3. The low density polyethylenefilms contained various amounts of iron, sodium chloride, bentonite clayand SAP as tabulated below with the weight ratio of SAP to iron heldconstant at a value of 0.5:1. Tabulated below is the amount of oxygenabsorbed by each of the film samples as measured by the OxygenAbsorption Test described above at test conditions of 168 hr, a relativehumidity of 100% and a temperature of 22° C. This example demonstratesthat for low density polyethylene films containing iron, SAP and sodiumchloride at a given weight ratio of SAP to iron, sodium chlorideincreased the oxygen-scavenging capacity of the low density polyethylenefilm and that as the amount of sodium chloride was increased, the oxygenscavenging capacity of the film also increased.

    ______________________________________                                                                              cc O.sub.2                              Film No.                                                                             Iron, %  NaCl, %  SAP, % Clay, %                                                                             gm Fe                                   ______________________________________                                        4-1    5.56     0.28     2.78   1.39  33                                      4-2    5.33     0.67     2.67   1.33  56                                      4-3    5.13     1.03     2.56   1.28  60                                      4-4    4.00     3.00     2.00   1.00  92                                      ______________________________________                                    

EXAMPLE 5

Concentrates of the ingredient mixtures of Example 4 and polymer resinwere prepared at a 50/50 weight ratio with linear low densitypolyethylene granules (GRSN 7047, Union Carbide) by tumble mixing thecomponents in a bucket/bottle roller for ten minutes to obtain ahomogeneous mixture. The resulting blends were formed into pellets bythe procedure described in Example 2 and the concentrates were mixedwith low density polyethylene pellets (Dow 5261, Dow Chemical) in a 1:4weight ratio and these pellet blends formed into films foroxygen-scavenging testing. The films were tested at conditions of 168hr, a relative humidity of 100% and a temperature of 22° C. The amountof thermoplastic polymer in the film was 90 weight % and thecompositions of the remaining components are tabulated below togetherwith the oxygen absorbed. This example demonstrates that theoxygen-scavenging composition of this invention comprising athermoplastic resin, iron, sodium chloride and SAP provides equivalentor better oxygen absorbing efficiency than the thermoplastic resin, ironand sodium chloride, with or without clay. Comparative compositions witha thermoplastic resin, iron, sodium chloride and SPH or SPA all exhibitconsiderably lower oxygen absorption values. Also, comparativecompositions with no electrolyte compound, sodium chloride, present allexhibited very low values of oxygen absorption. The water of hydrationof the SPH led to processing difficulties during film extrusion.

    ______________________________________                                                                      Additive,                                                                            Clay,                                                                              cc O.sub.2                          Film No.                                                                             Fe, %  NaCl, %  Additive                                                                             %      %    gm Fe                               ______________________________________                                        5-1    5.00   3.75     SAP    1.25   0    54                                  5-2    4.44   3.33     SAP    1.11   1.11 40                                  Comp 5-A                                                                             10.0   0        --     0      0    0.3                                 Comp 5-B                                                                             5.71   4.29     --     0      0    23                                  Comp 5-C                                                                             5.00   3.75     --     0      1.25 27                                  Comp 5-D                                                                             5.00   3.75     SPH    1.25   0    4                                   Comp 5-E                                                                             5.00   3.75     SPA    1.25   0    5                                   Comp 5-F                                                                             8.00   0        --     0      2.00 1                                   Comp 5-G                                                                             8.00   0        SAP    2.00   0    3                                   Comp 5-H                                                                             8.00   0        SPH    2.00   0    0.6                                 Comp 5-I                                                                             8.00   0        SPA    2.00   0    0.5                                 ______________________________________                                    

COMPARATIVE EXAMPLE A

Comparative, extruded low density polyethylene films were prepared byextruding a mixture containing 80 pbw low density polyethylene pellets(DOW 526 I, Dow Chemical) and 20 pbw of concentrates prepared accordingto Example 2 with various amounts of citric acid tripotassium salt("CATP") as the additive. Citric acid tripotasssium salt upon waterabsorption has a pH of 9. The extruded films were prepared according tothe method described in Example 3 with the films having nominalthicknesses of 5 mils. The amounts of oxygen absorbed by the filmsamples as measured by the Oxygen Absorption Test described above attest conditions of 168 hr, a relative humidity of 100% and a temperatureof 22° C. are given below. This comparative example demonstrates thatcitric acid tripotassium salt, having a pH greater than 7 upon waterabsorption, when added to NaCl is ineffective in enhancingoxygen-scavenging properties. Comparative films B-3 and B-4 with onlySAP or sodium chloride as the additive exhibited oxygen absorptionvalues of 3 and 26 cc O₂ /gm Fe, respectively.

    ______________________________________                                                                                   cc O.sub.2                         Film No.                                                                             Iron, % NaCl, %  CATP, %                                                                              SAP, %                                                                              Clay, %                                                                             gm Fe                              ______________________________________                                        B-1    4.44    3.33     1.11   0     1.11  0                                  B-2    4.00    3.00     2.00   0     1.00  1                                  B-3    5.71    0        0      2.86  1.43  3                                  B-4    5.00    3.75     0      0     1.25  26                                 ______________________________________                                    

EXAMPLE 6

Low density polyethylene films were prepared by extruding a mixturecontaining 80 pbw low density polyethylene pellets (DOW 526 I, DowChemical) and 20 pbw of a concentrate prepared according to Example 2with various amounts of nicotinic acid ("NIT") and sodium chloride.Nicotinic acid upon water absorption has a pH of 4-5. The extruded filmswere prepared according to the method described in Example 3 with thefilms having nominal thicknesses of 5 mils. The amount of oxygenabsorbed by the film samples as measured by the Oxygen Absorption Testdescribed above after 168 hr at a relative humidity of 100% and atemperature of 22° C. is tabulated below. This example demonstrates thatnicotinic acid in combination with sodium chloride can improve oxygenscavenging ability and that nicotinic acid without the electrolytecompound, sodium chloride, was not effective in increasing the oxygenscavenging ability of the composition.

    ______________________________________                                                                            cc O.sub.2                                Iron, %  NaCl, %  Clay, %     NIT, %                                                                              gm Fe                                     ______________________________________                                        4.00     3.00     1.00        2.00  49                                        5.71     0        1.43        2.86   4                                        ______________________________________                                    

EXAMPLE 7

Low density polyethylene oxygen scavenging films were prepared byextruding a mixture containing 80 pbw low density polyethylene pellets(DOW 526 I, Dow Chemical) having a nominal OPC of 1.5-2.1×10⁻¹⁰cc-cm/cm² -sec-cm Hg, as measured at a temperature of 20° C. and arelative humidity of 0%, and 20 pbw of concentrates containing variousamounts of iron, sodium chloride, bentonite clay and SAP as tabulatedbelow in the manner described according to Example 2. The film wasprepared using a Haake Rheomex 245 single screw extruder (screwdiameter-19 mm; UD ratio-25:1). The zone temperatures of the extruderbarrel were set as follows: zone 1--245° C., zone 2--250° C., zone3--250° C. and die--230° C. The extruded films had nominal thicknessesof 5 mils. The amounts of oxygen absorbed by the film samples asmeasured by the Oxygen Absorption Test at test conditions of 168 hr, arelative humidity of 100% and a temperature of 22° C. are given below.This example demonstrates good oxygen absorbtion performance even at lowlevels of electrolyte plus acidifying components but that oxygenabsorption was erratic at low electrolyte to acidifier ratios. Thelatter results are believed to have been caused by difficulties ineffectively mixing the compositions with low levels of sodium chloride.

    ______________________________________                                                                            cc O.sub.2                                Iron, %  NaCl, %  SAP, %      Clay, %                                                                             gm Fe                                     ______________________________________                                        5.6      0.3      2.8         1.4   55                                        6.5      0.3      1.6         1.6   69                                        7.1      0.4      0.7         1.8   50                                        7.4      0.4      0.4         1.9   44                                        7.6      0.4      0.2         1.9   49                                        5.7      0.06     2.8         1.4   45                                        6.6      0.07     1.7         1.7   19                                        7.4      0.07     0.7         1.8   29                                        7.6      0.08     0.4         1.9   15                                        7.8      0.08     0.2         2.0   46                                        ______________________________________                                    

EXAMPLE 8

Low density polyethylene oxygen scavenging films were prepared byextruding a mixture containing 80 pbw low density polyethylene pellets(DOW 526 I, Dow Chemical) having a nominal OPC of 1.5-2.1×10⁻¹⁰cc-cm/cm² -sec-cm Hg, as measured at a temperature of 20° C. and arelative humidity of 0%, and 20 pbw of concentrates prepared accordingto Example 2 with iron, bentonite clay, citric acid and sodium chloride.Upon water absorption, citric acid has a pH of 1-2. The films wereprepared according to the method described in Example 3 with theextruded films having nominal thicknesses of 5 mils. The amounts ofoxygen absorbed by the film samples as measured by the Oxygen AbsorptionTest described above at test conditions of 168 hr, a relative humidityof 100% and a temperature of 22° C. are given below. This exampledemonstrates that with an acidifier compound of high acidity, the amountof oxygen absorbed was significantly increased.

    ______________________________________                                                                   Citric        cc O.sub.2                           Additive                                                                              Iron, %   NaCl, %  Acid, % Clay, %                                                                             gm Fe                                ______________________________________                                        0       5.00      3.75     0       1.25   26                                  Citric Acid                                                                           4.44      3.33     1.11    1.11  174                                  Citric Acid                                                                           4.00      3.00     2.00    1.00  197                                  ______________________________________                                    

EXAMPLE 9

Two separate concentrate preparations of various oxygen scavengeringredients were carried out in the following manner: In oneconcentrate, iron powder (SCM iron Powder A-131); sodium chloride(Morton pulverized salt, Extra Fine 325); and bentonite clay (Whittaker,Clarke & Davis, WCD-670) were mixed in a high intensity Henschel mixerin a weight ratio of Fe:NaCl:bentonite clay of 4:3:1. The mixedingredients were fed at a 50:50 by weight ratio with linear low densitypolyethylene powder (Dowlex 2032, Dow Chemical) to a Werner & PfleidererZSK-40 twin-screw extruder to form concentrate pellets. A secondconcentrate of 25 weight percent of anhydrous sodium acid pyrophosphate,(Sigma #7758-16-9) with linear low density polyethylene powder was alsoprepared in a ZSK-40 twin-screw extruder. Films of polyethyleneterephthalate ("PET") (nominal OPC of 1.8-2.4×10⁻¹² cc-cm/cm² -sec-cmHg), polypropylene ("PP") (nominal OPC of 0.9-1.5×10⁻¹⁰ cc-cm/cm²-sec-cm Hg), low density polyethylene ("LDPE") and linear low densitypolyethylene ("LLDPE") with various combinations of the aboveconcentrates were extruded. In all of the films, the weight ratio ofsodium chloride to iron was held constant at 0.75:1. The amounts ofoxygen absorbed by these film samples as measured by the OxygenAbsorption Test at test conditions of 168 hr, a temperature of 22° C.and a relative humidity of 100% are tabulated below.

    ______________________________________                                                                                cc O.sub.2                            Resin   Fe, %    NaCl, %  SAP, %  Clay, %                                                                             gm Fe                                 ______________________________________                                        PET     5.00     3.75     0       1.25  10                                    PET     4.00     3.00     1.00    1.00  14                                    PET     4.00     3.00     2.00    1.00  14                                    PP      5.00     3.75     0       1.25  28                                    PP      4.00     3.00     1.00    1.00  46                                    PP      4.00     3.00     2.00    1.00  50                                    LLDPE   5.00     3.75     0       1.25  39                                    LLDPE   4.00     3.00     1.00    1.00  99                                    LLDPE   4.00     3.00     2.00    1.00  98                                    LDPE    5.00     3.75     0       1.25  29                                    LDPE    4.00     3.00     1.00    1.00  41                                    LDPE    4.00     3.00     2.00    1.00  48                                    ______________________________________                                    

EXAMPLE 10

Two separate concentrates were prepared by the same procedure as inExample 9. One concentrate consisted of iron powder (SCM iron powderA-131); sodium chloride (Morton pulverized salt, Extra Fine 325);bentonite clay (Wittaker, Clarke & Davis, WCD-670); and linear lowdensity polyethylene resin (Dowlex 2032, Dow Chemical) in a weight ratioof Fe:NaCl:bentonite clay:LLDPE of 4:3:1:8. The second concentrateconsisted of anhydrous sodium acid pyrophosphate (Sigman #7758-16-9) andlinear low density polyethylene (Dowlex 2032, Dow Chemical) in a weightratio of SAP:LLDPE of 1:3. Low density polyethylene oxygen scavengingfilms were prepared by the same procedure as described in Example 3using a Haake Rheomex 245 single screw extruder. The film processingtemperatures varied from nominal 470° F. to nominal 500° F. to nominal550° F. At nominal 470° F., the zone temperatures of the extruder barrelwere set as follows: zone 1--465° F., zone 2--470° F., zone 3--470° F.and die--425° F. At nominal 500° F., the zone temperatures of theextruder barrel were set as follows: zone 1--490° F., zone 2--500° F.,zone 30--500° F. and die--450° F. At nominal 550° F., the zonetemperatures of the extruder barrel were set as follows: zone 1--540°F., zone 2--545° F., zone 3--550° F. and die--485° F. At the higherprocessing temperatures, the resulting films were found to contain voidsbelieved to have been caused by decomposition of sodium acidpyrophosphate. Thermal gravimetric analysis of sodium acid pyrophosphatepowder heated from room temperature to about 1130° F. at a rate of about18° F./minute indicated weight loss occurring from about 500° to 750°F., corresponding to loss of water from sodium acid pyrophosphate, thussuggesting decomposition thereof to NaPO₃. Based on these observations,it is believed that the higher processing temperatures used in thisexample led to decomposition of the sodium acid pyrophosphate that wasoriginally used to sodium metaphosphate, sodium trimetaphosphate, sodiumhexametaphosphate, each having a pH in the range of 4-6 in aqueoussolution, or a combination thereof. The amounts of oxygen absorbed bythese film samples as measured by the Oxygen Absorption Test at testconditions of 168 hr, a temperature of 22° C. and a relative humidity of100% are tabulated below.

    ______________________________________                                              Nominal                                                                       Film                                                                    Film  Processing                                                                              Iron,    NaCl,                                                                              SAP,   Clay,                                                                              cc O.sub.2                          No.   Temp., °C.                                                                       %        %    %      %    gm Fe                               ______________________________________                                        10-1  470       4.44     3.33 1.11   1.11 40                                  10-2  550       4.44     3.33 1.11   1.11 53                                  10-3  500       11.11    8.33 2.78   2.78 48                                  10-4  550       11.11    8.33 2.78   2.78 77                                  ______________________________________                                    

We claim:
 1. An oxygen-scavenging composition comprising an oxidizablemetal component, an electrolyte component and sodium acid pyrophosphate.2. The oxygen-scavenging composition of claim 1 wherein the oxidizablemetal is iron.
 3. The oxygen-scavenging composition of claim 1 whereinthe electrolyte component comprises sodium chloride.
 4. Theoxygen-scavenging composition of claim 1 further comprising awater-absorbant binder.
 5. The oxygen-scavenging composition of claim 1further comprising a polymeric resin.
 6. The oxygen-scavengingcomposition of claim 5 comprising about 5 to about 150 parts by weightof the oxidizable metal plus electrolyte plus acidifying components perhundred parts by weight of the polymeric resin.
 7. The oxygen-scavengingcomposition of claim 6 wherein the polymeric resin comprisespolypropylene.
 8. The oxygen-scavenging composition of claim 6 whereinthe polymeric resin comprises polyethylene.
 9. The oxygen-scavengingcomposition of claim 6 wherein the polymeric resin comprisespolyethylene terephthalate.
 10. The oxygen-scavenging composition ofclaim 5 in the form of a concentrate in a thermoplastic resin.
 11. Theoxygen-scavenging composition of claim 5 in the form of a fabricatedarticle.
 12. An oxygen-scavenging composition comprising iron, sodiumchloride and sodium acid pyrophosphate wherein the weight ratio ofsodium chloride to sodium acid pyrophosphate is about 5/95 to about 95/5and about 5 to about 150 parts by weight sodium chloride and sodium acidpyrophosphate are present per hundred parts by weight iron.
 13. Theoxygen-scavenging composition of claim 12 further comprising awater-absorbant binder.
 14. The oxygen-scavenging composition of claim13 further comprising at least one thermoplastic resin.
 15. Theoxygen-scavenging composition of claim 12 further comprising at leastone thermoplastic resin.
 16. An oxygen scavenger component concentratecomposition, suitable for use in melt processing thermoplastic resinformulations comprising an oxidizable metal component, such concentratecomposition comprising at least one thermoplastic resin and at leastabout 10 parts by weight sodium acid pyrophosphate per hundred parts byweight thermoplastic resin.
 17. An oxygen-scavenging compositioncomprising at least one oxidizable metal selected from the groupconsisting of iron, zinc, copper, aluminum and tin, an electrolytecomponent and a non-electrolytic, acidifying component, in a thermosetresin.
 18. An oxygen-scavenging composition comprising at least oneoxidizable metal selected from the group consisting of iron, zinc,copper, aluminum and tin, an electrolyte component, a non-electrolytic,acidifying component and a water-absorbent binder in a polymeric resin.19. An oxygen-scavenging composition comprising at least one oxidizablemetal selected from the group consisting of iron, zinc, copper, aluminumand tin, an electrolyte component and a non-electrolytic, acidifyingcomponent, in the form of a concentrate in a thermoplastic resin.